Stamp sensing letter sorter



May 17, 1960 G. J. HARMoN STAMP sENsING LETTER soRTER Filed Oct. 5, 1954 3 Sheets-Sheet 1 May 17, 1960 G. J. HARMON STAMP sENsINc: LETTER somma 3 Sheets--Sheei'I 2 Filed Oct. 5, 1954 I N VENTOR ATTORNEY May 17, 1960 G. J. HARMON 2,936,886

STAMP SENSING LETTER SORTEIR Filed Oct. 5, 1954 3 Sheets-Sheet t, to t, tzu

fa i'5 21,91

INVENTOR Gregory d Harmon ATTORNEY United States Patent a STAMP SENSING LETTERSORTER Gregory J. Harmon, 'Bethesda,`Md., assign'or -to -Reed Research, Inc., Washington, D.C., faxcorporatlon of :Delaware Application October 5,1954, Serial No. 460,385; v 9 claims. .tomos-411) This invention relates to adevicefor sorting mailin ,accordance with Athe presence or `absence Aof a postage .stamp thereon, and more specifically to a photoelectiic sensing system for controlling Athe operation of selector .gates to automatically separate properly stamped letters :from vunstamped letters.

Various attempts have vbeen'made to provide-a practical stamp sensing optical system for sorting mail by sensing `systems assume a clean, highly reilective envelopesurface which `is optically homogeneous, and anV optically sharply contrasting stamp surface. In practice, however, these favorable conditions are seldom found to exist.

`Practically, mail often contains '.discolored or otherwiseY marked surfaces which vgive signals undistinguishable in a naive system from those produced by stamps. Furthermore, present day postage stamps are of many different colors and shades, and envelopes are often of very `similar colors and shades. Wrinklesand other surfaceiirregu-larities of the envelope also `tend to` produce'changes in the reflective condition of the envelope which closely resemble the changes produced bya stamp. A still greater difficulty exists in the case.ofpicturexpostcards, one entire side of which Vis usually covered with optically contrasting marks, which produce signals usually indistinguishable from thoseproduced by :a stamp.

It is a primary object of my inventiontoprovidea stamp sensing system which takes into `consideration'a'll of the -above enumerated difficulties 4and operates Vwith an extremely high percentage of accuracy. More specifically, it is lan object of the inventiontoprovide a stamp sensing system capable of distinguishing an unstamped picture postcard'from one-whichis stamped. A further object of the invention liswto provide a stamp sensing system `which minimizes 'the effects of small irregularities ofV an envelope such vvas wrinkles, printing and other small marks, ap edge, etc.,while providing a maximum signal for a stampi.

According to the invention, the abovev and other results Yare'obtainednby the .use ofa differential optical'system which 'compares :contiguous velemental 'reflected vareasof the envelope or other itemA ofmail,:andwhich is gated for operative sensing only :of that'areaipf the 'envelope surface where :a stampis normally placed. inhibiting vmeans are provided responsive Vto fopticalirregularit-ies of other areas onwthe mail whereby a postcard or similar -ptically mottled surface is prevented .from .triggering the selector gate. A special rtypeof balanceddifferp v2,936,886 Patented May 17, 1960 ICC AThe specific nature of my invention as lwell as other objects and advantages thereof will clearly -appearfr'om a description of a preferred embodiment as shown in the accompanying drawings, vin which:

Pig.' 1 is a schematic circuit diagram of sensing arrangement;

Pig. 2 is a detailed circuit diagram of this same arrangement;

Fig. 3 is atiming chart, showing the time relationship of the various pulses. occurring in the circuit of Fig. '2;

Figjll is a schematic plan View showing 'the principle the .base-edge ,of the letter selector gates;

.Fig.'"5 .'zsV a .detailed view o-f the gates shown vinl'lig.` 4;

Fig.' Gris a schematic circuit diagram of the 1front-edge vsensei; v

Fig.' 7 is a front elevation of the -photocell'housing unit; *and Fig. -8 is a sectional plan view of the same .unit,'jshow 4Ving the same relationship of the light beams and vletter surface.

Referring to'Fig. l, la piece of mail suchas an envelope jorpostcard'Z, which vis assumed tobe bearing a stamp 4 Vmeansform not part ofthe present invention, and'will `notbe described further. The unit to be `described 'is'a zp'hoto'ele'ctric* unit generally; indicated Vat 8 lfor optically scanning a `limitedarea of the letter 2 adjacent its bottom edge i't5-as the letter passes in iront of the scanning unit 1.8,v ewh'ich comprisesttwo "light sources 1t) and 12 so arranged las to reflect equal amountsof light iux tol'respective phototubes-M vand i6 when no stamp 'is present fin -the area'and when the reilecting surface Vofthe letter is optically homogeneous, that is, reasonablyclearand uniform. "Light is `admitted to photocells `14! and 16 vthrough two Ynarrow slits lSan'd 20 runningparallel'v to edge 2 of the letter and upward irom bottom edgef ithereof. In practice, the slits are formed Iin an opaque V-maskf'directly in yfront of the photocells, andloc'ated 'adjacentfland parallel to 'the surface vof the letter. Hence, the photocells'will conduct in equal amountwhen'any 'planesurface of uniform reflectance passes near to-and 'parallel to .the plane `of 4the mask,to cause reilection from areas land 20', corresponding to the areas of slits AAV18 -and 20 respectively.

A third photocell 22 is so arranged with respect toa 'light ibeamfrom lamp 2.4VA and fwith respect to diiferential arrangements, lthatthewback edge 3 has just passed light 'beam 23 ywhen slits i8 and 20 are apposite point 26 of the letter. .Light beam :'23, previouslyl obstructed by *the letter, :'canf-now reach photocell 22 ytoinitiatea desired `sequenceof'operations which will be vfully described `below. .The .time ofthis-'event will henceforth be termed tp It will be noted that light beam 23 was previously interrupted byzfrontedge 2 ofthe letter at a time which willsbe designated f1tTiandwhich :isz'of no particular sig# `nificance for our purpose. .A short time after 1T, "the '.fron'tzedge also. reachesscanner -SLand begins to Aaffect theascanner; this time will `beref'erred toas to andliis `lalso .of no :particular-signiiicance except as a reference point. .The important sequence of events which affect -the'operation yofthesystem-begins at time t1 Thej principle `of operation of 'the inventionwill now be described with `reference to Fig. l; it will be understood '.thatFig. vl is a vhighly schematic diagram intended only `:toi illustrate".thelunderlying principles and general mode .'side.

the system to indicate the time sequence of events. Initial interruption of light beam 23 by front edge 2 of the letter causes an increase of voltage at time tT, while subsequent resumption of the light beam causes a decrease of the voltage at time t1. This is inverted in amplifier inverter 32 and the resultant positive-going pulse at time t1 produced on line 33 triggers square wave gate generator 34 to produce a square pulse on line 35 whosev duration, as determined by the time constants of the gate generator extends until time t2. This pulse is used on line V356 to energize the lower half of ip-flop 42; it is also used to initiate another pulse at delay gate generator .38 which appears on line 39 at time t3 and-lasts until sensed by scanner 8 will not be passed through gated amplifier 40. Therefore, the gated amplifier passes only what the scanner sees on the area of the letter reserved for stamps. Assuming that a stamp is positioned in the proper place and is sensed by scanner 8, the output of Ygated amplifier 40 on line 41 is a positive pulse extending between times t3 and t4. This pulse is fed into the upper unit of dip-flop 42 to energize line 43, and deeuergize line 45 if that was previously energized, and thereby energize magnet 46 to operate mechanical gate 48 to direct letter 2, which has just been sensed, into channel I50 reserved for all stamped letters having this particular orientation.

In the absence of a definite signal from the scanner 8, the lower half of fiip-op 42 remains energized and supplies line 4S to energize magnet 47 to direct lthe mail into channel 52. It will be noted that the lower half of flip-flop 42 has previously been energized by the pulse extending from time tl-tz on line 36 as previously described.

The above system is in itself operative for ordinary letters to direct them into channel 52 if no stamp signal ,-is received, and to direct them into channel S is a stamp signal is received. It will be noted that there are four possible orientations of the letter. That is to say, the letter may be fiipped about either its horizontal or its 'vertical'axis, and in each of these positions the stamp may be on the side facing the scanner or on the opposite Therefore, assuming a random distribution of the mail, and assuming practically all of the letters to be properly stamped, as is usually the case, substantially one quarter of the letters will be delivered to channel 50 and all of these will be faced the same way. A similar arrangement to that shown in Fig. 1 can also be used to scan the other side of the letters, but in this case the stamp must be looked for near the front edge. If all letters were of the same size, the top edge of the letters .could be scanned by a similar arrangement suitably spaced from the bottom edge. However, due to the differences in sizes of letters, it is necessary to use the bottom edge as a reference. For this reason, postal stamp cancelling machines are also arranged to operate at a given distance from the bottom edges of letters. It is therefore necessary, after both sides of the bottom edge of the letters have been examined, and no stamp found, to turn the remaining letters over and run them through a similar arrangement on the edges which were previously the top edges. This can either be performed by hand, or by suitable known machinery, which is no part of the present invention. All of the letters not sorted by the machine will continue on a separate. channel and will ultimately require inspection by a human operator, For

the machine to Abe practical and effective, this residue must be reduced to a minimum and should constitute only a very small percentage of the original mail.

In practice, particularly during the summer time, a large percentage of the mail consists of picture postcards, the picture face of lwhich will usually produce a signal indistinguishable from that due to the presence of a stamp. It Will be noted that this occurs on the unstamped side of the postcard, since the stamp is always required to be placed ,on the blank or writing side. If the side which is being scanned is the picture side, it should not be passed as a stamped piece of mail, but `should be rejected, and the present invention provides means for ac complishing this. Essentially, this consists in supplying an inhibiting gate for the upper side of iiip-iiop 42, whereby if the dierential scanner 8' has been producing a substantial signal output in the area immediately preceding the stamped area, then selection due to a signal impulse jatthe stamp area is inhibited. Ultimately, of course,

the stamped side will be scanned and will produce a proper signal to channel the postcard for segregation with other mail similarly faced.

The postcard inhibiter gate generator 54 is supplied .over line 56 with the output of differential amplifier .37.

Itis gated on over line 36 by a pulse from time tl-tz.

vIt is therefore affected by the output of the differential amplifier from point 26 (Fig. 1) on. If the picture side of a postcard is being scanned, there will be a definite output from the area which causes gate generator 54 `to produce an output on line 58 which is made to have a duration from time t1-t4 so as to inhibit the operation of magnet 46. Thus the postcard is not passed through channel 50 but continues on channel 52, even though a signal has `been produced by the stamped area due to the markings thereon.

While the above device has been described as `a postvcard inhibiter, it will be apparent that it will also inhibit the selection of any marked envelope surface due to printing, advertising, etc. In practice, it has been found that the area immediately preceding the stamp area on a piece of mail is nearly always free from objectionable markings which would interfere with the proper operation of the device.

Fig. 2 is a schematic circuit diagram of a practical circuit embodying the invention of Fig. l. Corresponding elements have been given the same reference char- -acters as in Fig. 1, but, of course, it will be necessary to. employ a number of additional reference characters to identify'the details not shown in Fig. 1. The place- Yment of symbols tT through t5 with respect to the diagram of the letter shown in Fig. 2 represents the relative position of a point on scanner with respect to the letter as the letter passes by. Thus, at tT, the leading edge of the letter has not yet reached the scanner; at time to, the leading edge reaches the scanner and begins to affect it; and

so on, as described in connection with Fig. 1. The wave lforms appearing on the various conductors of the circuit are again shown and are identified by capital letters so that their timing relationship may be more readily correlated with the corresponding waveform shown in Fig. 3.

In the following discussion, the amplifier unit 37 will be referred to as a differential amplifier because this expression best indicates the essential function of the amplifier; although it is not a conventional differential amplifier, its output is indicative of the differential reflectance'of the'areas 18 and 20. This results from the 'fact that the left side tube has a conventional plate re sistor, which may conveniently be in the order of K, While the right side tube operates essentially as a cathode follower and has no plate resistor. Thus the current Vflowing in the left tube is greater than that flowing Ythrough the iight tube for the same energization of the rphotocells, and the adjustment is such that the plate voltage at B is related to the differential reectance. In practice, this arrangement, for which no novelty vis 'ciaimedgper se,fhas been found to -work verysatisfactorily. f

Thefinput from one side ofsCa-nner 8 to the correspondings'ide of differential amplifier-37 isshown at A. IItwill be Yunderstoodv that asimilar waveform appears at' lineV 16 but is displaced intime from waveform A l'by a-time-period At corresponding tothe timefrequired for "the passage offthe mail--betweeny slots '-18 and 20. ySince-the width of each slot-is approximately of the same order -as the distance between slots, `it will --be apparent `that the output from each slot representsan average effect of a certain area, and the differential outptitgfi'om amplifier;37 represents-the vdifferenceofthese average effects. -It Ais clear, however, that-when one slithas just tpassed the edge of a stampfwhile the other uslit -s still Yon -the envelope surface, lthere will be a 4sharp rise-in output. kThis occurs at time t3, and,due tothe discontinuities offthe optical surface' of :the stamp, -an irregular'output of moderate Vv alve will-'continue until `the `trailing edge Qfi the vstamp, passes the scanner 8 at which time 'the Aoutput will fall to-its original level. I have "found empiricallyfthat exact-balancing of thedifferential amplifier is undesirable, and lappears v`toemphasize noise, that is,-the signal due tovary smalll variations of the optical surface of the letter. By-unbalancingathe anode resistors -offthe differential amplifier, a definite increase in the signal-tonoise ratio hasbeen noted. --In effect,'1itfappearsthat the spikes-due to all signalsV are reduced vl-by a-fairly constant amount, vwhich tends to suppress the small -signals,-while scarcelyaffecting the large-'signals 'upon -which the operationof the device depends. On the -other hand,- accurately' balancing the circuit appears -to emphasizenhe small discrepancies. 'Thee-optimum effect appears'to be obtained, inthe circuit shown, by making the gain for one side ofV the differential amplifier approximatelybne- -`half of the gain for the other side.

-Continuing with the description of :the circuit, the output `of the differential amplifier, Ahaving the waveform `:B (Figs. 2 and 3) yis -supplied to ygated amplifier-40. This amplifier is gated by a-delayed gate D on 'line 3-1, as in AvFig. 1,-putout by delayedgate generator38, during the `time -intervalftS-tg If'lthere-is a stamp' in the rdetected area, 'then during this interval amplifier 40 lwill -put'rout aA signal as shown atfE. vThis -signal "is pa-ssed'-through -pulse `transformer 60, and is also inverted as shown -at by connecting the grid of thyratron 42a to the `transformer secondary terminal of 'the properpolarity to -accomplishthis inversion. The secondary ef Ytransformer Ailis-shunted Vwith a 15,000 ohmfresistor- GAZ-for impedance lmatching and -circuit clamping, to Vminimize ringing. Thyratron 42a constitutes the -vupper Yhalf of flipvflop\42 of Fig. l, which `is accomplished by coupling its -anode through lead 64 and condenser 66 to the anodeV of thyratron 42b, `which constitutes the lower half of-the iiiplflop. Therefore, if tube 42a is'fired -while 42b nis on, fthe-consequent lowering vof -potential at its yanode will depress still further the plate voltage of thyratron'42b, `and cause it to be extinguished, asis well known. `Ener gizationis thereby switched from magnet A47 to magnet -i6-as explained in connection vwithFig. l. vAt the time this occurs, tube 42B Awill vnormally .be on1becauseLit--is fired -by .an earlier-timed pulse, as will'now be explained.

The gating signal forrthe above procedure is timed` by the end of the interruption of light beam 23 by the letter as previously explained. This-producespulseH in lline, 3) which is invertedlinamplifier'32fasshownxatfl This pulsel is differentiated bytheccombinatiom of ;con .denser 70 kand resistor72 shownfas afpotentiorneterV for adjustability, to produce spikes at tT and t1 Aas `shown .at 1. As thyratron 7,4 is normally deionized;thefnegative spike tT-has no effect, but the positive spike `-at t1 fires thyratron 74-to produce a waveformfasfshown -at1K, ses denser 76 discharges through .the ionized-:tube :74 the deionizing-voltage is f-reached andthe :tube

senses" tcccnduct. .Waveform-K ist sharpened by the-.RQ 15 I combination'` of condenser 78 and resistorf'80,-fandfedfto -the grid of gate generator 34, extending from :time tI--tg `as 'determined by the time constant of the preceding rRC combination.l This pulseis fed on line.36 to gate the picture postcard inhibitor as explained in connection with Fig. l, and is also fed online to normallyconducting delayed gate tube .38, where the :negative-going side of the pulse at time t2 again cuts 4oi tube/38fdue to the clamping action of the grid of tube-38, fora` time extending from t2-t4 as determined bythe :timeconstant of the RC combination in the grid circuit of` tube-38, similar to thatfor tube 34. The output of ltube 38on -line 39 `is pulse D extending from t2-.t4 which `is used as the gating pulse for amplifier 40 as previously explained. It will be noted that by virtue of thislarrangement, time t2 and t3 occur at the same instant, that is, Sthey become a single time. This is done primarilyfor convenience and to simplify the circuitry. T-Sofar as the principle of the invention is concerned, time interval l--tz could be selected to fall on any suitable area of the letter in advance of the postage stamp area.

V-Pulse M `from tube '34 is also fed, through potentiometer-79, to the primary of transformer 80, the secondary of which is shunted by suitable 15K resistor, similar to resistor 62 of transformer 60, and used to fire thyratron `42b at time t1. This 'insures that thyratron 42b is fired in advance of 42a.

Pulse M is also Ifed along line 36, as previously fexplanned, to one control grid of tube-82 which functions as a gated amplifier for the picture postcard inhibitor. The other control grid of tube 82 is fed through line 56 Awith pulse B from the differential amplifier 37. If vthe 4waveform is as lshown at B, nothing happens, since-the lpositive signal -at time t3 has. not yetbegum However, if the scanner 8 has been: scanning the-picture side offa picture postcard, orits optical equivalent, then'A the differential amplifier 37 will be producing a signalprior to time t3, and this signal Will be gated throughamplier 82-by gate pulse M. Ordinarily, this pulse will extend in duration from t1-t2 and passes from tube '82l along the lline 83 to pulse transformer 84, where its polarity is reversed and it appears at the grid of thyratron 54v as pulse' Q, to produce an output on line 58 as pulseiR, which inhibits tube 42a as previously explained. Thus, :it is seen hoW the circuit of Fig. 2 accomplishes all offfthe functions generally described in connection with the schematic diagram of Fig. l.

It will be understood that an arrangement similar to scanner 8, at a different station on the conveyer linel carrying the letters, is used to scan the opposite face ofthe letter `from that scanned by scanner 8, except thaty the reference time must be taken from the front edge'of'the letter instead of Ifrom the back edge, as Vwill be explained below. If noy stamp` is found, the letter is flippedl over Yin vany suitable manner, and continued along 'the conveyer now resting on edge 5 as its bottom edge, which Vwas previously its top edge. It will be notedv that in one of these positions, the stamp may'appeary adjacent-the front' edge of the letter rather than adjacent Vthe back `edge as shown in Figs. l and 2. `For this position, the scanner shown in Fig. l and Fig. 2 Vwill produce no 'result, since it is scanning the wrong corner of the letter. Vvit is therefore necessary, for two of the sensing stations, to trigger the action of the scanner by the front-edge of the letter rather than by the rear edge andn in this .case to Vplace the scanner immediately behind the-light beam V2.3 (or its equivalent) so vthat it will scan the proper-area. Alternatively, the rejected letters could be turned topedge down and returned tothe conveyer system at a point ahead of the same two stations for front and back edge scanning. While this would save two stations,fit :would also reduce the speed of the system, sov that the useof four stations is generally preferable. However,

' ftheii-ght beam,'instead;of being resumed as beforeibycthe bacio-edge, is now interrupted bythe fronte edge, .a andthe timing pulse is reversed in polarity compared to the preyious situation. It is therefore now necessary to reverse the grid connections to the differential amplifier and to reverse gates 34 and 38 in order to secure the same action as before with the reversed polarity. Furthermore, since the picture postcard gate now occurs after the stamp area has been scanned and not before it as in the previous case, it is no longer possible to inhibit thyratron 42a as before. An inhibiting pulse must be fed to thyratron 42b to trigger this thyratron at the correct time, as will be explained below, and consequently deionize thyratron 42a to operate the correct letter door relay 47. EX-

`cept for these readily apparent changes, the circuit is the same as before.

Fig. 6 shows how the above results are accomplished.

.Except for the above noted differences, the circuit of Fig. 6 is the same as that of Fig. l, and corresponding elements are identified by the same reference numerals with prime added. It will be noted that the letter is shown moving in the opposite direction to that of Fig. l, since the possibility of a stamp being located adjacent the front edge is now being investigated. Light beam 23 is interrupted by front edge 2 of the letter to produce the same kind of wave shape as before, since the polarity of the photocell 22 is reversed from that shown in Fig. 1. As the leading edge of this pulse is the only point of interest, the pulse is differentiated by the condenser-resistor arrangement shown at 99' and fed to inverter amplifier 32', whose output is a sharp negative pulse at time to which is fed to square wave generator 3.4. The output of generator 34 is a square wave lasting from to to l2 3. The back end of this pulse at time t3 is fed to delayed gate 3S' and used to produce a delayed pulse output lasting `from t3 to t4. However, this pulse is not supplied to gated amplifier 4th as before, but is now supplied to one grid of coincidence tube S2 while the first gating pulse from generator 34 is now supplied to the gated amplifier 40'. It will thus be seen that leads 36 and 39 have been interchanged, which is necessary because scanner 8 is looking atrthe stamp area during time t1 2, instead of time t3 4 las before. If there is an output from gated amplifier 37 during time t1 2 corresponding to a stamp (or a picture postcard), this output will appear on line 41 to trigger unit 42a' of the flipop and operate the accept gate. The flipflop was previously triggered to 42h by the pulse at time t0 passing Ialong line Miti to Winding SWA of transformer Sti', which is now provided with two primary windings Sti'A and SWB. Y After this triggering of 42h', if there is no output from the differential amplifier, representing no stamp, magnet 47 remains energized. Note that tube 54 and transformer 84 of Fig. 2 are now omitted, Since the delayed inhibiting pulse is not needed with the leading-edge senser.

In the ,case of a picture postcard, unit 42a' will be triggered as though by la stamp, during the interval t1 2; however, during the interval t3 4 the picture postcard will produce a signaloutput on line 56 from the differential amplifier which will be gated through tube 82' by the delayed gate on line 39 and will reset the fliptlop to the reject position through winding B of transformer 80. Thus, instead of causing the picture signal to inhibit as in the previous case, it is caused to reset the flipflop in the leading edge senser. This enables the above-described simplification of the circuit since the output of tube .82' can now merely be fed to a second primary winding of Atransformer S as no pulse-extending circuit is now re- `of the letter edge opposite the two slots will'not produce.

.entire gate.

an unwanted differential signal resembling a stamp signal due to the contrast of the black background and reflected light. The angle of beam 23 must therefore be Vsuch that this beam is not interrupted until after the letter has safely passed both slits of the mask before the gate is produced. Therefore, to actually represents the time when both slits have been covered by theletter. In the back edge senser, similarly, the delayed gate must definitely be terminated before either of the two slits clears the back edge of the letter, which is controlled by proper selection of the time constants in the gate generators.

While a schematic showing of -a mechanical gate `suitable for channeling the lettersris shown in Fig. 1 as 'a pivoted gate arrangement directly operated by the respective magnets 46 and 47, the speed at which such a gate can be operated is found to be rather slow in com1 parison with the desired speed. A more practical gate for high speed operation is shown in Figs. 4 and 5. In this case, as before, the problem is to direct the front edge ,of letter 2 into the selected one of channels 50' and S2'.

This is done` by means of two pivoted selector gates 96 and 98. Normally these gates are held latched in the position shown by respective latches 92 and 93. Assuming magnet 47 to be energized, it attracts armature 90 of latch 9210 unlatch selectorgate 96. Gate 96 is still `held in the position shown by the action of tension spring V97. While the latch 92 is so released, the letter 2 is advanced against the gate at a fairly high speed (for example, at ten vfeet per second) by a conveyor system shown schematically as a belt and pulley system. Selector gate 98 being latched cannot be moved by the letter, but gate 96 has been unlatched and is therefore moved aside by the letter into the position shown in dotted lines and Athe letter passes by its own inertia into channel 52. Conversely, if magnet 46' is energized, the letter will pass into channel 50. After the letter has passed through, spring 97 resets selector gate 96 t6 its original position as shown and when magnet 47" is deenergized, latch 92 is again reengaged to secure the gate against motion. Since the magnets are now required only to condition the gates by moving a small latch member for a very short distance, it will be apparent that much faster operation is possible than with the sort of arrangement shown in Fig. l, where the magnets are required to move the As shown in Fig. 5, the adjacent ends of gates 98 and 96 have an interlocked comb-toothed construction, to facilitate guiding the letter into the proper channel.' In practice, it has been found that the gate construction as shown in Fig. 4 can handle approximately ten letters a second.

Figs. 7 and 8 show the physical construction of the photocell housing. In practice, instead of two separate cells 14 and 16, a single envelope double photocell 15 is used, which is placed in an opaque tubular housing 17 provided with slits 18 and 20, as in Fig. l. Photocell 22 is also placed in the same housing at a higher level, as

vshown in Fig. 7, facing approximately at an angle of 45 to the plane of the letter, and a suitably placed light beam source 24 (Fig. l) is directed at cell 22 at an angle so that the point of interception of the beam by the edge of the envelope occurs in the correct relationship with the position of the slits 18, 20, to produce the desired timing sequence.

It will 'be apparent that the embodiments shown are .onlyexemplary and that various modifications can be made in construction and arrangement within the scope 'of the invention as defined in the appended claims.

lbysensing the position of the stamp, comprising means for conveying the randomly oriented letters in single file 'past a series of four photoelectric scanning devices for .7.5 -4producing an 'electricaloutput in response to the reflectance of the letter surface scanned, each optical device being positioned to scan a different potential stampbearing corner-*area and a normally homogeneous adjacent area of each letter as it passes the device, a selector gate associated with eac-h scanning device for selecting a letter from the moving le when said gate is energized 1 by a predetermined level of electrical output from its two longitudinal and two vertical edges with one longitudinal edge lying in a datum line and moving `along said line, means for producing a beam of light across the path of said letter, photoelectric means in the path of said beam responsive to the passage of the back vertical edge of said letter past a station to produce an electric timing pulse, a stationary scanner comprising means for illuminating two adjacent areas of one side lofvsaid letter and two photocells, each positioned and masked to receive light from a different one of said areas only, said areas each having a vertical extent from said datum line corre sponding to the height of a stamp in the lower back corner of the letter, a differential amplifier connected to said photocells for producing an electrical output which is a function of the differential reflectance of said'areas, first pulse gate means electrically connected to said photoelectric means for producing a first gating pulse timed by said timing pulse to commerce when the scanner is positioned to scan a reference area of said letter adjacent the back corner and to end by the time the scanner is positioned to scan said corner area, second pulse gate means for producing a second gating pulse timed to substantially coincide with the scanning of the corner area, an electrically controlled letter selector gate having one electrical state for electing said letter and a second electrical state for rejecting said letter, first coincidence gating means for producing said one electrical state, said first coincidence gating means being connected to the output of said differential amplifier and to said second pulse gate means, and means controlled by the output of said first pulse gate means for producing said second electrical state.

3. The invention according to claim 2,y said first coincidence gating means being connected to produce said one electrical state upon coincidence of gating input Y signals from said differential amplifier and from said second pulse gate means.

4. The invention according to claim 3, including second coincidence gating means connected to the output of said differential amplifier and said first pulse gate means for inhibiting said one electrical state, said second coincidence means being biased to produce an inhibiting pulse upon` coincidence of signals from said differential amplifier and` said first pulse gate, saidV inhibiting pulse commencing from the beginning of saidv rst gating pulse, and means for extending said inhibiting pulse through the entire duration of said second gating pulse, said inhibiting pulse being applied to said letter selector gate to inhibit said one electrical state.

5. The invention according to claim 2, said diferential amplifier comprising two amplifiers having differentially related outputs and means for unbalancing said outputs by an amount sufficient to suppress the efect of small ditferences in refiectance of said areas but to pass large signals such as are'produced by thepresence of a stamp,L

, 10 whereby the effect of small optical differences is minimized. f

6. Means for conveying a rectangular letter having two longitudinal and two vertical edges with one longitudinal edge lying in a datum line and moving along said line, means for producing a beam of light across the path of said letter, photoelectric means responsive to the passage of the front vertical edge of said letter past a station to produce an electric timing pulse, a stationary scanner comprising means for illuminating two adjacent areas of one side of said letter and two photocells, each positioned and masked to receive light from a different one of said areas only, said areas each having a vertical extent from said datum line corresponding to the heightof a stamp in the lower front corner of the letter, a differential amplifier connected to said photocells for producing an electrical output which is a function of the differential reflectance of said areas, first pulse gate means in circuit relationship with said photoelectric means for producing a first gating gate having one electrical state for selectingsaid letterY and a second electrical state for rejecting said letter, first coincidence gating means for producing said one electrical state, said first gating amplifier meansVv being connected to the output of said differential-amplifier and to said first pulse gate means and means'controlled by the beginning of said timing pulse for producing said second electrical state.

7. The invention according to claim 6, said first coincidence gating means being connected to produce said one electrical state upon coincidence of gating the input signalsV from said differential amplifier and from said first pulse gate means. v

8. The invention according to claim 7, and second coincidence gating means for producing said second electrical state, said second coincidence gating means being connected to produce said second electrical state upon coincidence of signals from said differential amplier'and from said seond pulse gate means, whereby reception of a signal from said reference area causes said letter selector gate to be in the reject position.

9. The invention according to claim 8, said differential amplifier comprising two amplifiers having differentially related outputs and means for unbalancing said outputs byv an amount suficient to suppress the effect of small differences of reectance of said areas but to pass large signals such as are produced by the presence of a stamp,

whereby the effect of small optical differences is mini- Y mized.

References Cited in the le of this patent I .UNITED STATES PATENTS 1,217,092A Hopp Feb. 20, 1917 1,729,521 Roddy Sept. 24, 1927 2,224,646 Friedman et al. Dec. l0, 1940 y 2,280,948 Gulliksen Apr. 28, 1942 Y 2,395,482 Hurley Feb. 26, 1946 2,415,177 Hurley Feb. 4, 1947 2,609,928 Doust ..-a Sept. 9, 1952 

